Conventionally, wireless tire condition monitoring apparatuses have been proposed that permit the driver in the passenger compartment to check the conditions of the tires of a vehicle. One such tire condition monitoring apparatus includes a receiver mounted in a vehicle body and transmitters attached to respective wheel assemblies. Each transmitter detects the condition of the associated tire, that is, pressure and temperature in the tire, and wirelessly transmits a signal containing data that represents the detected tire condition. The receiver receives signals transmitted from the transmitters through a reception antenna, and displays information related to the tire conditions on a display provided in the passenger compartment as necessary.
In such a tire condition monitoring apparatus, the receiver is preferably configured to determine which one of the wheel assemblies a received signal has been transmitted from, in other words, the position of the wheel assembly associated with the received signal.
In the tire condition monitoring apparatus disclosed in Patent Document 1, a transmitter is attached to each of the wheel assemblies of a vehicle. The transmitter includes a pressure sensor configured to detect the air pressure in the tire and a transmission circuit configured to incorporate information related to the tire air pressure detected by the pressure sensor into a signal (radio wave) and transmit the signal. A receiver is provided in the vehicle body. The receiver has a reception antenna, which is arranged at different distances from the respective transmitters. The receiver includes a reception circuit configured to measure the received signal strength indication (RSSI) of the signal and a reception-side control section configured to output the RSSI measured by the reception circuit. Signals transmitted from the transmitters are attenuated before reaching the reception antenna. Thus, the greater the distance from the reception antenna to the transmitter, the weaker the received signal strength indication, or RSSI, of the signal received by the receiver becomes. The reception-side control section identifies the positions of the wheel assemblies based on the magnitude relationship of the differences between the maximum values and the minimum values of the RSSIs of the signals transmitted from the transmitters. For example, the receiver determines that the ascending order of the magnitudes of the difference between the maximum value and the minimum value of the RSSI agrees with the ascending order of the distances between the reception antenna and the respective transmitters that have transmitted the signals. Based on the positional relationship with the transmitters, the receiver determines the positions of the wheel assemblies, each of which has a transmitter.
Also, the reception-side control section obtains the air pressures of the respective tires based on the information related to the tire air pressure contained in the signals transmitted from the transmitters. If there is an abnormality in the air pressure of any of the tires, the reception-side control section notifies the driver of the abnormality.